What Causes Delayed Periods After Having An Abortion?

due to missed abortion i had my D&C on january 8th 2010 and second D&C on january 16th because of the leftover tissue.i didnt have my periods till may 6th.the doctor then adviced to go on for the next curettarization as they found it to be a case of haematometra.Now it has been almost a month I did not have my periods. My husband and me have regular sex with precautions.Could you pls let me know the reasons for the delay or missed periods.

Thanks